1. Field of the Invention
This invention relates to a method and apparatus for promoting flow of finely divided solids material which flow is at least over one or more transitory time intervals a "plug" flow. By the term "plug flow" in this specification is meant flow of a finely divided solids material in counter-current flow to a liquid in which the flows of liquid and solids material are maintained such as to each occupy substantially homogeneously the cross-section of flow and such as to be at a separate substantially constant flow speed at all points at such cross-section.
2. Prior Art
The establishment of "plug" flow is essential or desirable in certain manufacturing processes particularly in processes for refining of metals or other crystalline substances. Thus, for example, U.S. Pat. Nos. 2,540,997, Re 24,038 and 2,540,083 describe purification of low-melting organic substances wherein a slurry of the crystalline substance produced by preliminary chilling of the liquid substance is fed into the top or central portion of a column which is heated at the base and has provision for removal of purified liquid substance from the base of the column and for recycling the impurity enriched liquid from the top of the column. Again, U.S. Pat. No. 4,043,803 there is described a process for continuous refining of metals which process comprises forming a liquid column of the metal, said column having a relatively cool zone and a relatively hot zone and a continuous temperature gradient between said zones; the temperature of the cool zone being such as to permit coexistence of liquid and crystalline metal and said hot zone being at a temperature sufficient to melt the crystals; partially melting a solid massive body of the metal to form a slurry of the crystalline metal in the liquid metal, feeding said slurry into the cool zone of said column; transporting the crystals through the temperature gradient and into the hot zone, wherein melting of the crystals occurs and the hot zone thereby becomes enriched with the pure liquid metal and continuously withdrawing a portion of the liquid metal from the hot zone.
With both of these processes, if good results are to be obtained in terms of throughput, the flow of crystals within the column must be, over at least a transitory time period, plug flow in counter flow to the flow of liquid metal. The efficiency of the processes is related to the rate of flow of the crystals and at least for appreciable production rates there must be a substantial rate of flow. On the other hand, the greater the flow rate the more difficult it becomes to properly maintain the desired form of flow, there being a tendency for the liquid material to develop channel flow at particular locations through the advancing crystals or other material.
The prior art discloses various means for facilitating achievement of the desired flow characteristics. For example, it is known to "pulse" the column by pulsing of liquid at the foot of the material in the column to cause periodic movement thereof. McKay, Dale & Weidman have described (Industrial & Engineering Chemistry volume 52 No. 3 at Page 157) an arrangement in which an inverted cup-shaped piston covered at its lower end with a mesh screen is positioned within the column material and reciprocated. Australian Pat. No. 416,845 discloses arrangements in which a stirrer is positioned within the column and rotated. The stirrer is in the form of an elongate shaft having a plurality of sidewardly extending projections. Whilst these arrangements are to some extent satisfactory, they do not produce as great an improvement in efficiency in some operations as may be possible and it is an object of this invention to provide improved means for facilitating attainment of the aforementioned desirable flow characteristics.